Stop 6:Puzzling Patterns

Here are some enigmatic markings on the underside of a piece of siltstone lying on the creek bed, eroded from one of the formations upstream. Such markings, called fucoids, were once believed to be the remains of marine algae (seaweed), but despite careful microscopic examination, no organic structures have ever been detected in these impressions. The ridges we see here are actually casts of furrows made in the soft mud (later turned to shale) that has since eroded away. But what made them? They could be casts of crustacean tracks, worm burrows or mollusk trails, but no one knows for sure.